Mestrado Integrado em Medicina Dentária

Faculdade de Medicina da Universidade de Coimbra

Microtensile dentin bond strength evaluation of different adhesive

systems: in vitro study

Sandra Filipa Seabra de Campos

Orientador: Prof. Doutor João Carlos Ramos

Coorientador: Dr. Fernando Marques

Coimbra, 2013

Agradecimentos

Ao meu orientador, Professor João Carlos Ramos. Os seus profundos conhecimentos nesta área

nortearam de forma decisiva este trabalho. A Sua dedicação, a atenção dispensada, a amizade,

a sua determinação durante todo este ano, constituem um exemplo que procurarei seguir

durante toda a minha vida. Um Bem-haja!

Ao Dr. Fernando Marques, o meu sincero agradecimento pela coorientação deste projeto. Muito

obrigada pelo profissionalismo, pela sincera amizade, total disponibilidade e confiança

depositada em mim durante toda a realização deste trabalho.

Dra. Alexandra Vinagre da mesma forma por todo o estímulo durante a realização deste trabalho.

Além de toda a contribuição intelectual, obrigada também pelo carinho e amizade.

À Dra. Ana Messias por toda a ajuda na análise estatística dos resultados.

Ao João Sousa pela ajuda na realização do esquema apresentado no trabalho.

A todos os Professores do Mestrado Integrado em Medicina Dentária pelos conhecimentos, pelo

apoio e confiança durante todo o curso. Foi um privilégio frequentar este Mestrado, nesta

Faculdade, que muito contribuiu no enriquecimento da minha formação académica, científica e

também cívica.

A todos os meus colegas (e amigos) de curso por toda a sinceridade, companheirismo e acima

de tudo amizade durantes estes tão ricos anos académicos.

À minha família, em especial aos meus Pais, Irmãos e Avós, um enorme obrigada por

acreditarem sempre em mim e naquilo que faço e, acima de tudo, por todos os ensinamentos de

vida. Espero que esta etapa, que agora termino possa, de alguma forma, retribuir e compensar

todo o carinho, apoio e dedicação que, diariamente, me oferecem.

Às minhas sobrinhas Soraia, Bruna e Sofia, meus tesouros, pelos sorrisos e pelo carinho que me

oferecem todos os dias.

Aos meus cunhados, um grande obrigada pela paciência e pela amizade que sempre tiveram

comigo.

Aos meus padrinhos e primos Francisco, Paulo e Joana por toda a força.

Às minhas grandes amigas, Sis, Sandrina, Filipa, Gi, Marlene, Fi e Rita, pela amizade, pela força

e presença nos bons (e menos bons) momentos.

Às minhas colegas de casa e amigas, Lurdes Veloso, Rita Tavares, Teresa Duarte e Catarina

Sousa pela forma como me acolheram e integraram em Coimbra. Obrigada pela amizade,

companhia e afeto.

2

Microtensile dentin bond strength evaluation of different adhesive

systems: in vitro study

Campos, S.; Marques, F.; Ramos, J. C.

Área de Medicina Dentária, Faculdade de Medicina, Universidade de Coimbra

Av. Bissaya Barreto, Blocos de Celas

300-075 Coimbra

Portugal

E-mail: sfsdcampos@gmail.com

3

Abstract

Introduction: Modern restorative dentistry has been extensively influenced by the

fast progress in dental adhesive technology. To assess the effectiveness of adhesive

systems and to predict their clinical performance the resin-dentin bond strength tests has

been widely used.

Purpose: The goal of this study was to compare the microtensile bond strength of

five different adhesives systems at the dentin-composite interface and evaluate failure

modes.

Materials and methods: Flat dentin surfaces were prepared in 25 non-carious

human molars. Exposed dentin surfaces were wet-grounded with 240-, 400- and 600-grit

silicon-carbide sandpaper to create bonding surfaces with a standardized smear layer. The

teeth were randomly divided into 5 distinct groups according to the adhesive system tested:

three self-etching adhesive systems, Xeno® V+ (Dentsply DeTrey, Konstanz, Germany);

Xeno® III (Dentsply DeTrey, Konstanz, Germany) and ClearfilTM SE Bond (Kuraray Medical

Inc., Okayama, Japan), and two etch-and-rinse adhesive systems, OptiBondTM FL (Kerr,

Orange, CA, USA) and Prime&Bond® NT (Dentsply DeTrey, Konstanz, Germany) applied

with respect to manufacturer’s instructions. After adhesive procedures a 4-mm thick

composite crown was built over the bonded surface. Following the storage in distilled water

at 37 ºC, the samples were vertically cross-sectioned until obtaining sticks with 1.37mm2 of

cross-sectional area which were tested in tension in a universal testing machine at 0.5

mm/min. Data were analyzed by one-away ANOVA and a Tukey HSD post-hoc test

(p<0.05). The mode of failure was also analysed with optical microscopy. Additionally, dentin

disks were obtained, treated with the different conditioners and primers and observed by

scanning electron microscopy (SEM).

Results: The following microtensile bond strengths were registered (mean in

MPa±SD): Group I – Xeno® V+ 3.70±5.01; Group II - Xeno® III 18.94±13.87; Group III –

OptiBondTM FL 43.29±12.74; Group IV- Prime&Bond® NT 39.64±15.06 and Group V –

ClearfilTM SE Bond 42.80±10.65. Etch-and-rinse (OptiBondTM FL; Prime & Bond® NT) and

two-step self-etching (ClearfilTM SE Bond) adhesive systems attained significant higher

microtensile bond strength, without statistically significant differences between them.

Significant lower values were obtained for Xeno® V+ comparing to Xeno® III. Cohesive

4

composite failures were related with the higher bond strength values, whereas the adhesive

failures were associated with the lowest bond strength values.

Conclusion: Among the materials evaluated, etch-and-rinse and two-step self-

etching adhesive systems presented higher dentin bond strength than the one-step self-

etching adhesive systems.

Keywords: microtensile; dentin bond strength; etch-and-rinse; self-etch; dentin

adhesion

5

Introduction

Improvements in dental adhesive technology have extensively influenced modern

restorative dentistry1. Today’s, operative dentistry should primary involve “minimally invasive”

techniques1, 2 that promotes a more conservative cavity design, which basically relies on the

effectiveness of current enamel-dentine adhesives1.

Bonding to enamel has been demonstrated to be easy and durable while bonding to

dentin is far more challenging3, due to the heterogeneity of structure and composition of

dentin, its surface characteristics after bur cutting and chemical treatments1, 3-7 and relation

with pulpal tissue by means of numerous fluid-filled tubules1. Basically, current dentin

adhesives employ two different means to achieve the goal of retention between restorative

material and dentin8: by removing the smear layer with etch-and-rinse adhesives or

modifying the smear layer with self-etch adhesives.

The adhesion to tooth substrate implies an exchange process in which some

inorganic tooth material is removed and replaced by resin monomers2, 9 that, upon

polymerization, become micromechanically interlocked in the created microporosities10, 11 ,

rather than on a primary chemical adhesion12. However, this twofold bonding mechanism is

believed to be advantageous in terms of restoration longevity13,14.

Contemporary dental adhesive systems can be classified according to the application

techniques as etch-and-rinse and self-etching adhesives systems15. In the first one, the acid-

etching application completely removes the smear layer, followed by the application of primer

and bond resin in one or two steps16. The etch-and-rinse technique is still the most effective

and stable approach for enamel bonding2. However, concerning dentin bonding, this

technique can be considered to be difficult and less predictable1. With the self-etching

adhesive systems the clinician can simplify the clinical procedures and also reducing clinical

time17, therefore to reduce technique sensitivity or risk of making errors during application

and manipulation2. Nevertheless, some potential problems are related with the use of some

self-etching adhesives, like postoperative sensivity, incomplete marginal seal, premature

bond degradation, biocompatibility, and compromised bonding to abnormal substrates18.

The self-etch technique does not require a separating etching step, it uses acidic

monomers that simultaneously etch and prime the dental substrate1. The self-etch adhesive

systems can be classified according to the pH of the adhesive solutions in strong (pH<1),

intermediately strong or moderate (pH between 1.0 and 2.0) and mild (pH>2)2. The high

acidity results in rather deep demineralization effects. So, the bonding mechanism of ”strong”

self-etching adhesives can be similar to the etch-and-rinse approach2. However, low-pH self-

etch adhesives reveals low bond strength values, especially at dentin2 with a high number of

pre-testing failures, especially when tested with a microtensile methodology2.

6

The aim of this study was to compare the dentin microtensile bond strength of five

adhesive systems.

The null hypothesis was that there were no significant differences between the five

adhesives systems evaluated.

7

Table I: Adhesive systems studied, manufacturers, chemical composition, pH values and batch

numbers.

Materials and methods

Specimen preparation

Twenty-five non-carious human molars were collected and stored in distilled water

within 10 weeks after extraction. The teeth were cleaned from debris and partially included

in an acrylic resin block (Orthocryl®, Dentaurum). The oclusal surfaces were cut

perpendicularly to the long axis of the tooth (Accutom 5, Struers, Ballerup, Denmark), under

water-cooling, thereby exposing a flat dentin surface without residual enamel. All oclusal

surfaces were wet-ground with a sequence of 240-, 400- and 600-grit silicon-carbide

sandpaper in circular motion for 60 seconds to obtain a uniform smear layer. The prepared

oclusal surfaces were carefully observed using an optical microscope at a 40-fold

magnification (M300, Leica, Switzerland) to confirm the absence of residual enamel or

another defects in dentin surfaces.

Bonding and restorative procedures

The teeth were randomly divided in five groups, according the five adhesive systems

tested: two and three-step etch-and-rinse adhesives and one and two-step self-etch

adhesives (Table I).

Adhesive Manufacturer Chemical Composition pH Batch no.

Group I

Xeno® V+

1-step/ 1 bottle

Self-etch Adhesive

Dentsply DeTrey,

Konstanz, Germany

-Bifunctional acrylate -Acidic acrylate - Functionalized phosphoric acid ester -Water -Tertiary butanol -Initiator - Stabilizer

1.319

1203000016

Group II

Xeno® III

1-step/ 2 bottles Self-etch Adhesive

Dentsply DeTrey,

Konstanz, Germany

Liquid A: -HEMA; purified water; ethanol; BHT; highly disperse silicone dioxide Liquid B: -Pyro-EMA; PEM-F; urethane dimethacrylate; BHT; camphorquinone; ethyl-4-dimethylaminobenzoate

1.42

1302000019

Group III

OptiBondTM

FL

3-step Etch-and-rinse

adhesive

Kerr, Orange, CA, USA

Etchant: 37.5% phosphoric acid Primer: -HEMA;GPDM; PAMM; ethanol; water; photo initiator Adhesive: - TEGDMA; UDMA; GPDM; HEMA; bis-GMA; filler; photo initiator

1.82

4677483

8

BHT: Butylated hydroxyl toluene; Bis-GMA: Bisphenol A diglyciyl methacrylated; GPDM: glycerol

phosphate dimethacrylate; HEMA: 2-hydroxyethl methacrylate; PAAM: Phthalic acid monoethyl

methacrylated; PEM-F: Mono fluoro phosphazene modified methacrylate; PENTA: Dipentaerythritol

pentaacrylate phosphate; PYRO-EMA: Phosporic acid modified methacrylate; MDP:

methacryloyloxydecyl; TEGDMA: triethylene glycol dimethacrylate; UDMA: Urethane dimethacrylate;

The bonding and light-curing procedures were carried out as recommended by each

manufacturer (Table II).

Following the bonding procedures, the crowns of the cut teeth were

reconstructed with three incremental layers (1.5 mm) of light-cured microhybrid composite

resin Esthet.X® HD A2 (DentsplyDeTrey, Konstanz, Germany) (Table III). Each layer was

Group IV

Prime&Bond® NT

2-step Etch-and-rinse

adhesive

Dentsply DeTrey,

Konstanz, Germany

-Di-and trimethacrylate resins -PENTA -Photoinitiators -Stabilizers -Acetone -Nanofillers

2.2 2

1206000730

Group V

ClearfilTM

SE Bond

2-step

Self-etch Adhesive

Kuraray, Okayama,

Japan

Primer: 10-MDP; HEMA; hydrophilic aliphatic dimethacrylate; dl-camphorquinone; N,N-Diethanol-p-toluidine; water Adhesive: Bis-GMA; 10-MDP; HEMA; hydrophobic aliphatic dimethacrylate; dl-camphorquinone; N,N-Diethanol-p-toluidine; colloidal silica

1.92

041931

Group/ Adhesive system

Application procedure

I - Xeno® V+ Apply actively adhesive for 20 sec; air-drying for 5 sec; light-curing for 10 sec.

II - Xeno® III

Mixing equal amount of Liquid A and B for 5 sec; apply actively for at least 20

sec; air-drying; light-curing for 10 sec

III - OptiBondTM

FL

Apply 37.5% phosphoric acid (Kerr Gel Etchant®) for 15 sec; rinse for 15 sec;

gently air-dry; apply primer actively for 15 sec; gently air-dry for 5 sec; apply the

adhesive for 15 sec air-dry for 3 sec; light-curing for 20 sec.

IV - Prime & Bond® NT

Apply 36% phosphoric acid for 15 sec; Spray and rinse with water for 15 sec; blot

dry conditioned areas; apply adhesive and leave the surface wet for 20 sec;

gently air-dry for at least 5 sec; Polymerize for 10 sec; apply a second layer of

adhesive in similar way.

V - ClearfilTM

SE Bond Apply primer for 20 sec; mild air stream; apply bond; gentle air stream; light-

curing for 10 sec.

Table II: Adhesive application procedures (according manufacture recomentations).

9

polymerized separately for 10 seconds followed by an extra-time final polymerization of 60

seconds (Bluephase®, Ivoclar Vivadent, Lichenstein).

Composite Manufacturer Composition Filler Batch no.

EsthetX HD

A2

Microhybrid

Dentsply

DeTrey,

Konstanz,

Germany

Bis-GMA adduct

Bis-EMA adduct

TEGDMA

Ba-F-Al-B-Si-glass

Nanofiller sílica

(77wt%; 60 vol%)

1006292

Bis-GMA: Bisphenol A dimethacrylate; Bis-EMA: Bisphenol A polyethylene glycol diether

dimethacrylate; TEGDMA: Triethyleneglycol dimethacrylate

Immediately after composite curing, the teeth were kept intact and stored in distilled

water at 37ºC during 7 days (Heraeus BK 6160, Kelvitron® Kp, Wehrheim, Germany).

Cutting method

The specimens were cross-sectioned perpendicularly to the adhesive-tooth interface

with a low-speed cutting saw (Accutom 5, Struers, Ballerup, Denmark), under water cooling

at 300 rpm, according to the technique described by Sano et al.20, to produce dentin-

composite resin sticks with a sectional square area of approximately 1.37mm2. After the first

cut in x-axis direction, the free residual space between the slices was filled with light-bodied

silicone Aquasil Ultra XLV (Dentsply, DeTrey, Konstanz, Germany) (figure 1). Finally, the

roots were cut from the crown approximately 2 mm bellow the cementoenamel junction

releasing the dentin/composite sticks which were then checked under a optical microscope

(M300, Leica, Switzerland) at 40-fold magnification in order to exclude samples with defects.

The number of stick specimens obtained per each group was: Group I (Xeno® V+) n= 27;

Group II (Xeno® III) n=24; Group III (OptiBondTM FL) n=41; Group IV (Prime & Bond® NT)

n=40 and Group V (ClearfilTM SE Bond) n= 37.

Table III: Composite resin; manufacturer, composition, filler and batch no.

Figure 1: Representative images of oclusal and vestibular view of the teeth, before the final

root section.

10

Microtensile bond strength testing

Each stick was bonded to a microtensile sample holder with cyanoacrylate adhesive

(Permabond® 735, PermabondInternational Co, Englewood, NJ) and then fixed on the

microtensile device (Od04-Plus; Odeme Dental Research, Luzerna, Brasil). Specimens were

fractured in tensile mode in a universal testing machine (Model AG-I, Shimadzu Corporation,

Kyoto, Japan) at a 0.5 mm/min speed and the maximum load (in MPa) at failure was record.

After the microtensile testing, the fractured sticks were examined with a optical

microscope (M300, Leica, Switzerland) at a 40-fold magnification and the mode of failure

was recorded.

Failures modes were classified as: adhesive, if total failure occurred within the

adhesive interface; cohesive in dentin (complete failure in dentin); cohesive in composite

(complete failure occurred in the composite resin) and, finally, mixed, when simultaneously

the adhesive and cohesive failure occurred.

Figure 2: Esquematic diagram from study

11

Ultramorphology analysis of dentin substrate by SEM

Two extra dentin disks of 1mm in thickness were obtained by means of two parallel

sections of a molar crown (Accutom 5, Struers, Ballerup, Denmark) and then wet-grounded

with a sequence of 240-, 400- and 600-grit silicon-carbide sandpaper in circular motion for 60

seconds to obtain a uniform smear layer, fixed in 2.5% glutaraldehyde within a PBS solution

for 24 hours and, finally, divided in four samples according the different dentin conditioning

preconized for each adhesive system: (1) 36% phosphoric acid; (2) ClearfilTM SE Bond

primer; (3) Xeno® III primer and (4) Xeno® V+. The samples were dehydrated in ascending

ethanol series of 50%, 75%, 95% and 100% for at least 2 minutes per step, except the last

one during for 4 minutes and immersed in hexamethyldisilazane (HMDS) until complete

solvent evaporation.

After chemical dehydration, the specimens were mounted on a specimen aluminium

stub using carbon adhesive, sputter-coated with gold-palladium (Polaron E-5000 Sputter-

Coater, Polaron Equipment Lta, Watford, U.K.) before SEM analysis with a Hitachi S-4100

microscope (Hitachi, Tokyo, Japan).

12

Results

Figure 3 and table IV shows the results of the microtensile bond strength of the five

adhesive systems tested.

OptiBondTM FL, ClearfilTM SE Bond and Prime & Bond® NT had the best performance

in microtensile bond strength test. “Spontaneous” interfacial debonding occurred in 11 cases

of the Xeno® V+ group, which presented inferior mean values. Additional similar pre-test

failures were only found in two samples of Xeno® III.

Group Adhesive Systems n Mean ±SD Min Max 95% CI

I Xeno® V+ 27 3.70±5.01

A 0.00 21.48 [1.72, 5.68]

II Xeno® III 24 18.94±13.87

B 0.00 42.55 [13.08,24.80]

III OptiBondTM

FL 39 43.29±12.74C 20.54 69.87 [39.16,47.42]

IV Prime & Bond® NT 40 39.64±15.06

C 13.35 67.44 [34.83,44.46]

V ClearfilTM

SE Bond 37 42.80±10.65C 18.36 66.84 [39.24,46.35]

Table IV: Descriptive statistics for microtensile bond strength values of groups. Mean, standard

deviation (SD), minimum and maximum values in MPa. Same upper case letters are not satistically

different.

Figure 3: Box plot graphic for microtensile bond strength values distribution within groups

13

Mean adhesion values for the five groups were compared using one-way ANOVA

setting the significance level at α=0.05 and considering zero as the value for pre-test failures.

Post-hoc analysis was performed with Tukey HSD multiple comparisons test. There were no

real outliers and the data was normally distributed for all groups (p>0.05) except Xeno® V+

(p=0.001), as assessed by box plot and Kolmogorov-Smirnov test. There was no

homogeneity of variances, as assessed by Levene’s Test of Homogeneity and Variance

(p<0.05). Microtensile bond strength values were statistically significantly different between

groups of adhesives F(4,162)=62.50, p<0.01, ω2=158.99. Bond strength values increased

from the Xeno® V+ group (3.70±5.01 MPa), to Xeno® III (18.94±13.87 MPa), to Prime &

Bond® NT (39.64±15.06 MPa), to ClearfilTM SE Bond (42.80±10.65 MPa) and to OptiBondTM

FL (43.29±12.74 MPa), in that order.

Post-hoc analysis revealed that the increase from Xeno® V+ to Xeno® III (15.24, 95%

CI [5.78,24.69]), from Xeno® V+ to Prime & Bond® NT (35.94, 95% CI [27.55,44.34]), from

Xeno® V+ to ClearfilTM SE Bond (39.10, 95% CI [30.57,47.63]) and from Xeno® V+ to

OptiBondTM FL (39.59, 95% CI [31.16,48.03]) was statistically significant (p<0,01). As shown

in Table V increases in mean bond strength values from Xeno® III to Prime & Bond® NT,

ClearfilTM SE Bond and OptiBondTM FL groups were also statistically significant.

There was a statistically significant difference between means (p<0.05), thus we can

reject the null hypothesis.

(I) Group (J) Mean Difference

(I-J) Std. Error p

95% Confidence Interval

Lower Bound Upper Bound

Xeno® V+ Xeno

® III -15.24

* 3,43 ,000 -24,69 -5,78

OptibondTM

FL -39.59* 3,06 ,000 -48,03 -31,16

PB® NT -35.94

* 3,04 ,000 -44,34 -27,55

ClearfillTM

SE Bond -39.10* 3,09 ,000 -47,63 -30,57

Xeno® III Xeno

® V+ 15.24

* 3,43 ,000 5,78 24,69

OptibondTM

FL -24.36* 3,17 ,000 -33,10 -15,61

PB® NT -20.70

* 3,15 ,000 -29,41 -12,00

ClearfillTM

SE Bond -23.86* 3,20 ,000 -32,69 -15,02

Table V: Multiple comparisons for groups I and II (Tukey HSD test). Mean difference of

microtensile bond strength values between groups in MPa

14

The crosstabulation, observed and expected frequencies for each cell of the design

are found in the Failure*Group Crosstabulation table (Table VI), as shown below:

Failure * Group Crosstabulation

Group

Total Xeno

® V+ Xeno

® III Optibond

TM FL PB

® NT

ClearfillTM

SE Bond

Failure

Ad Count 16a 13

b 0c 4c 2c 35

% within Failure

45,7% 37,1% 0,0% 11,4% 5,7% 100,0%

DC Count 0a 3a 2a 0a 4a 9

% within Failure

0,0% 33,3% 22,2% 0,0% 44,4% 100,0%

CC Count 0a 1a 31b 22b 25b 79

% within Failure

0,0% 1,3% 39,2% 27,8% 31,6% 100,0%

Mix Count 0a 5a 6a 14a 6a 31

% within Failure

0,0% 16,1% 19,4% 45,2% 19,4% 100,0%

Total Count 16 22 39 40 37 154

% within Failure

10,4% 14,3% 25,3% 26,0% 24,0% 100,0%

Each subscript letter denotes a subset of Group categories whose column proportions do not differ significantly from each other at the .05 level.

Chi-square test for association was conducted between failure type and adhesive

system. There was a statistically significant association between failure type and adhesive

system, Χ2 (12)= 112.64, p<0.05

There was a moderately strong association between failure type and adhesive system, V=

0.49, p<0.01. Adhesive failures were associated with both Xeno® V+ and Xeno® III. Cohesive

composite fractures were associated with both OptiBondTM FL, Prime & Bond® NT and

ClearfilTM SE Bond.

Table VI Failures mode percentage of the debonded specimens for each group: Ad - Adhesive

failure DC - Dentin cohesive failure; CC - Composite cohesive failure; Mix - Mixed failure;

Figure 4: Representative images of the different failure modes: Ad - Adhesive failure; CC -

Composite cohesive failure; DC - Dentin cohesive failure; Mix - Mixed failure.

15

Figure 6: Representative SEM images (6000X) of dentin treated with: (A)- 36% phosphoric acid

for 15 seconds and rinse with water; (B)- ClearfilTM

SE Bond primer; (C)- Xeno®

III; (D)- Xeno®

V+.

(A)

(BB)

(C)

(D)

SEM observations

Figure 5 is a SEM representative image of the smear layer adhered to dentin surface

and partially occluding the dentinal tubules before different adhesive dentin conditionings.

Concerning the ultra morphology analysis of the adhesive conditioned dentin, the

SEM analysis showed different patterns (Figure 6).

Figure 5: SEM representative image(6000x) illustrating the smear layer covered dentin.

(A) (B)

(C) (D)

16

The analysis of the dentin disks under SEM showed that after etching with 36%

phosphoric acid smear layer was completely removed and subjacent dentin and dentinal

tubule orifices were visible and their input was expanded.

The primer of two-step self-etching adhesive system, ClearfilTM SE Bond, removed

most smear plugs and showing the underlying dentinal tubules and also a partially visible,

dissolved smear layer in the intertubular area.

Unlike to phosphoric acid and ClearfilTM SE Bond primer, the patterns of dentin

surface created by one-step self-etch adhesive systems Xeno® III and Xeno® V+ showed that

smear plugs were not removed, or only partially, and a partially dissolved smear layer was

visible in the intertubular area.

17

Discussion

Today the current challenge in adhesive dentistry is to ensure the long-term success,

making the adhesive-tooth interfaces more resistant against aging2. One of the most

important issues of recent adhesive materials is its durability21, which seems to be dependent

either on the adhesive’s formulation and the bonding strategy21.

The transition between the restorative material and the dental hard tissue must be

continuous to increase the survival probability of the restoration22. In spite of adhesive bond

strengths to enamel are predictable, satisfactory and stable when etch-and-rinses systems

are employed23, 24 bonding to dentin is a much more complex issue, due to the nature of this

substrate24, namely its higher organic content and the presence of fluid and odontoblastic

processes in the tubules23. The long-term durability appears to be influenced by substrate

and polymer stability, activity of metal matrix proteins (MMPs) and the ongoing decalcification

by bacterial generated acids25.

No internationally standardization test protocols yet exists for the testing adhesives

systems26. Therefore, variable methods and parameters have been employed by the different

laboratories making difficult the comparison across studies27.

The interface between restoration and tooth is exposed to different challenges in oral

cavity such as forces that act in different directions and simultaneously2 or thermal

oscillations that can induce cumulative stresses and progressive interface degradation9. By

definition, the ideal bond strength test should be easy to perform, low technique-sensitivity,

relatively fast, unsophisticated and inexpensive7, and, most important, capable to simulate

biomechanics conditions of restored teeth. Bond strength testing is the method most used for

the assessment of bonding effectiveness to enamel and dentin28, 29 including different

mechanical methods30. These tests are used to evaluate the ability of a restorative material

or dentin bonding system to establish a bond to a biological substrate22.

Generally, bond strength can be measured by macro and micro test set-ups, basically

depending upon the bonded size area7. The bond strength tests results can be affected by

several parameters, like, operator, research group/institute, adhesive system, adhesive

class, substrate preparation, substrate origin and composites flexural modulus31. The

characteristic of the bonding substrate plays a major role on the quality of adhesion6.

Clinically relevant substrates include caries-affected, caries infected, sclerotic, deep and bur

cut dentin32. In the present study bond strengths were measured under a sound dentin

substrate that always play a important role in surrounding walls of cavities design. Besides

the same composite resin was used combined with each adhesive system studied for cutting

off the possible influence of resin composite modulus development.

18

The shear bond-strength test is the most commonly used30, namely between

manufacturers, because it is easy and fast to perform7. However, some problems have been

related to the shear bond strength test, as unrealistic stresses are produced within the

reaction zone31. Microtensile bond strength test allows measurements of the tensile bond

strength on very small surfaces, about 1 mm2, 20, 33. This method allows multiple specimens to

be prepared from each tooth34, measuring bond strength at critical areas31, with a more

uniform stress distribution in the reaction zone and a higher reliable correlation with clinical

retention loss31.

There are several critical factors that influence the results of the microtensile bond

strength test, such as diameter of the stick, type of jig, trimming35 and storage of bonded

specimens in water31.

In this test, the loading force passes through the tooth substrate and composite resin

before the adhesive interface36. Thus, the subsequent stress concentration could explain the

frequent cohesive failure in the tooth substrate36, which not reflect the true bond strength26.

Microtensile bond strength tests have a number of advantages, such as: more

adhesive failures and fewer cohesive failures27, 29; measurement of higher interfacial bond

strengths27,29; allows testing on very small surfaces27,29, 37 or in irregular surfaces27,29; means

and variances can be calculated for single teeth27, 29; and facilities examination of failed

bonds by scanning electron microscopy29,27. However, some disadvantages were also

described for the microtensile bond strength test, as the labour intensity, technical demand

and dehydration potential of these smaller samples27.

In the present study, the best performing adhesive systems was the 3-step etch-and-

rinse OptiBondTM FL (43.29±12.74), followed by 2-step self-etch Clearfil SETM Bond

(42.80±10.65) and Prime & Bond® NT (39.64±15.06) without no statistical differences found

between them. OptiBondTM FL has already showed very favourable laboratory38, 39 and

clinical performance28, 40, 41 Pashley et al4 referred that 3-steps etch-and-rinse adhesives are

more durable than 2-step etch-and-rinse adhesives, due to the first one has the advantage

and opportunity to use each step for essential multipurpose objectives.

OptiBondTM FL bonding resin composition has cross-linking monomers. The relative

amounts of Bis-GMA, TEGDMA and UDMA presents is this adhesive system may have a

meaningful influence on the viscosity of the uncured adhesive resin and on the mechanical

properties of the cured resin42. TEGDMA has high flexibility which is compensated by the

rigidity of Bis-GMA42. Some studies that used OptiBondTM FL presented very scatter bond

strength values. Phrukkanon et al43 (1998) obtained 20.2±5.0 MPa which is lower comparing

19

with the present study. Conversely, Heintze & Zimmerli26 (2011) in a study about the

relevance of in vitro tests of different adhesive systems, registered 48.0±13.7 MPa to

OptiBondTM FL, which is similar to those obtained in the present work. Along with two-step

self-etch adhesives, three-step etch and rinse adhesives are considered the gold standards

for dentin adhesion.

Presenting also good performance, Prime & Bond® NT, contains an acidic

phosphonated monomer (PENTA) which can interact with calcium ions left on dentin

surface21. This adhesive system is filled with nanoparticles that may help to establish a

thicker and more uniform resin film thickness that stabilizes the hybrid layer44. Prime&Bond®

NT had a mean microtensile bond strength of 39.64MPa. The performance of this adhesive

has been evaluated by other authors which registered lowers values than de present

study37,45.

Clearfil SETM Bond, though belonging to the group of “mild” self-etch adhesives9, also

had a high performance concerning bond strengths. This adhesive system was the only self-

etch obtaining microtensile bond strength values comparable to those etch-and-rinse

adhesives and no statistical differences were found between them.

Osorio et al.21 (2008) stated that high percentage of camphorquinone in this adhesive

might improve the degree of polymerization and the presence of 10-methacryloxydecyl

dihydrogen phosphate (10-MDP) which can contribute to the higher bond stability due to its

chemical adhesion with tooth tissues42. In fact, this 10-MDP molecule can chemically bond to

calcium of hidroxiapatite, according to the adhesion decalcification concept, forming a stable

calcium-phosphate salt, along with only a limited surface-decalcification effect. ‘Mild’ self-etch

adhesives indeed only superficially interact with dentin, and hardly dissolve hidroxiapatite

crystals, but rather keep them in place, forming a thin submicron hybrid layer3. In this study,

microtensile bond strength results for ClearfilTM SE Bond were 42.80±10.65 MPa. In the

literature, similar results were found to this adhesive system by other authors15,26.

Clinicians prefer materials with an easy, simplified application. However, these

simplified adhesives can be associated to a less optimal clinical effectiveness9.

In this present study the adhesive system with lowest performance was the Xeno® V+

(DentisplyDeTrey, Konstanz, Germany), followed by the Xeno® III (DentisplyDeTrey,

Konstanz, Germany). Xeno® V+ presented a mean of 3.70±5.01 (MPa) and 11 pre-test

failures within 27 specimens in total. Another study, by Nikhil et al.46 (2011), also

demonstrated a lower performance for Xeno® V+ and referred that the adhesive system bond

strength can be associated to absence and presence of HEMA, which is absent in case of

Xeno® V+ (HEMA-free). In adequate concentration, this monomer can contribute for bond

strength due to its hydrophilic nature that makes it an excellent adhesion-promoting

monomer and by enhancing wetting of dentin. Nevertheless, HEMA-free one-step adhesives

20

are complex blends of hydrophilic/hydrophobic ingredients, water and solvents, prone to

phase separation, which can account partially for their lower bonding effectiveness47. On the

other hand, Xeno® V+ adhesive system contains tertiary butanol, which is related to a more

stable formulation than those containing ethanol, according El-kholany et al48. Although a

long shelf-life has been attributed for this adhesive by the manufacturer, possible alterations

in Xeno® V+ components can also explain the lowest results reported in this study.

In this study Xeno® III also presented, comparatively, lower bond strength values. In

high amounts, HEMA, which is a Xeno® III component, may have deteriorating effects on the

mechanical properties of the resulting polymer42. For this adhesive system, Loguercio et al.49

reported a higher bond strength result, 28.3MPa, while Bortolloto et al.50 registered a mean

of 23.8 MPa, and Amaral et al.51 showed a mean of 19.9MPa in microtensile bond strength

for this one-step self-etch adhesive.

Simplified one-step self-etch adhesive system does not provide the formation of a

high quality hybrid layer compared to 2-step self-etching primers and conventional etch &

rinse adhesives15. One-step self-etch adhesives composition are a complex mixture of

hydrophobic acid monomer, hydrophilic resins, solvent and water15,29,52. The lack of

hydrophobic resins for hybrid layer formation15 and, consequently due to their hydrophilic

nature, these adhesives systems may act as permeable membranes and absorb significant

amounts of water, even when polymerized52 which can compromise bond strength to dentin

and influence the failure mode15. Moreover, monomer degradation has several adverse

effects on the performance of dental adhesives, predominantly by the deterioration of bond

strength and inducing morphological changes at the adhesive/dentin interface53.

In the presented study, the mode of failure was related with the adhesive system. The

adhesive failure was associated to low bond strengths values, meaning for the Xeno® V+ and

Xeno® III. On the other hand, the higher bond strength values were correlated with cohesive

failures that occurred in the best performance adhesive systems OptiBondTM FL, ClearfilTM

SE Bond and Prime &Bond® NT.

Similar results were reported in other study. Ceballos et al.44 (2003) correlated the

bond strength values with mode of failure. Low bond strengths were associated with

adhesive failures, while cohesive fractures were seen at higher bond strengths.

Scherrer et al.54 (2010) stated that the common cohesive failures that occurred in

microtensile bond strength tests may be due to the errors in the alignment of the specimen

along the long axis of the testing device or to the introduction of microcracks during cutting.

In the present study, cohesive failures occurred frequently, but correlated significantly well

with greater adhesion values. Concerning this point, it can be understood that, at least, the

adhesive bonding to dentin was stronger than the registered value and therefore of the

cohesive strength of resin or dentin.

21

Statistical analysis showed 11 pre-test failures within Group I (Xeno® V+) and 2 for

Group II (Xeno® III). The high number of the pre-test failures represented an additional point

for the scatter in microtensile bond strength results54. These values can be treated by

different manner by researchers31,54. In the present study they were considered as zero in the

statistical analysis, which explain the decrease in mean values and the increase in standard

deviation, but reflects truly the concern about the weakness of the produced adhesive

interface55.

Concerning the future research perspectives, it will be valuable improving the

standardization of test conditions, studying the same adhesives with different methodologies

and even with different operators, as well as conducting in vitro tests after aging,

complemented with clinical trials. Furthermore, researches evaluating other bonding

properties and different adhesive bonding approaches should be targeted.

22

Conclusions

Within the limitations of this in vitro study, it can be concluded that:

- Etch-and-rinse adhesives and two-step self-etch adhesives showed higher

microtensile bond strength values than one-step self-etch adhesives.

- Adhesive failure mode was correlated to lower bond strength results, while the

cohesive failures are more common with higher adhesion values.

Acknowledgments

The authors would like to thank the companies Dentsply DeTrey (Konstanz,

Germany), Kuraray Medical Inc. (Okayama, Japan) and Kerr (Orange, CA, USA) for the

donation of the materials used in this study.

23

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